Exhaust system of internal combustion engine

ABSTRACT

An exhaust system of an internal combustion engine includes a main catalyst converter, a bypass catalyst converter, a bypass catalyst converter with a smaller volume and a shift valve device. Upstream-side main exhaust passages have a collecting portion connected with a downstream-side main exhaust passage connected with the main catalyst converter. Bypass exhaust passages have the bypass catalyst converter therein, being connected between the shift valve device and the downstream-side main exhaust passage. The shift valve device is disposed at a position from the upstream-side portions of the upstream-side main exhaust passages to a head flange fixable to a cylinder head of the engine to be shiftable between a first shift position where the upstream-side main exhaust passages are closed and the bypass exhaust passages are opened and a second shift position where the upstream-side main exhaust passages are opened.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an exhaust system of internal combustion engine which is provided with a main catalyst converter and a bypass catalyst converter in order to purify exhaust gas discharged from the engine.

2. Description of the Related Art

A conventional exhaust system of an internal combustion engine is disclosed in Japanese Patent Application Laying-Open publication No. 2007-46556. This conventional exhaust system includes upstream-side main exhaust passages, downstream-side main exhaust passages, bypass exhaust passages, a main catalyst converter, bypass catalyst converter, and two shift valves. The upstream-side main exhaust passages are attached to exhaust ports of an internal combustion engine, and their downstream-side portions are connected with upstream-side portions of the downstream-side main exhaust passages through the two shift valves. The main catalyst converter is connected with downstream-side portions of the downstream-side main exhaust passages. The bypass exhaust passages connect intermediate portions of the upstream-side main exhaust passages and intermediate portions, between the shift valves and the main catalyst converter, of the downstream-side main exhaust passages with each other, and the bypass catalyst converter is disposed in the bypass exhaust passages. The bypass catalyst converter has volume smaller than that of the main catalyst converter.

When the engine is started, the shift valve closes the downstream-side main exhaust passages to prevent exhaust gas from flowing into the main catalyst converter, and it opens the bypass exhaust passages to flow the exhaust gas into the bypass catalyst converter. This accelerates rise in temperature of the bypass catalyst converter because of the small volume thereof, purifying the exhaust gas from earlier time.

The above known conventional exhaust system of the internal combustion engine, however, encounters a problem in that there is a room for improvement in the rise of heat in the bypass catalyst converter because of the following reason.

Exhaust gas is relatively low for a while immediately after the engine starts.

In the conventional exhaust system, junctions of the bypass exhaust passages are far away from the shift valves, so that considerable amount of the exhaust gas flows into portions of the upstream-side main exhaust passages between the junctions and the shift valves, consequently causing delay in heat-up of the bypass catalyst converter. It takes much time for the catalyst in the bypass catalyst converter to reach catalyst activation temperature.

It is, therefore, an object of the present invention to provide an exhaust system of an internal combustion engine which overcomes the foregoing drawbacks and can accelerate rise in heat in a bypass catalyst converter to purify exhaust gas as quick as possible immediately after an internal combustion engine is started.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided an exhaust system of an internal combustion engine including a main catalyst converter, a bypass catalyst converter, upstream-side main exhaust passages, a downstream-side main exhaust passage, bypass exhaust passages and a shift valve device. The main catalyst converter is used for purifying exhaust gas discharged from an internal combustion engine, while the bypass catalyst converter, also used for purifying the exhaust gas converter, has a volume smaller than a volume of the main catalyst converter. The bypass catalyst converter is arranged at a position closer to exhaust ports of the internal combustion engine relative to a position of the main catalyst converter. The upstream-side main exhaust passages include upstream-side portions and downstream-side portions having a collecting portion, and the downstream-side main exhaust passage has an upstream-side portion and a downstream-side portion, where the upstream portion thereof is connected with the collecting portion of the upstream-side main exhaust passages, and the downstream-side portion thereof is connected with the main catalyst converter. The bypass exhaust passages have upstream-side bypass passages and a downstream-side bypass passage so that the bypass catalyst converter is disposed between the upstream-side bypass passages and the downstream-side bypass passage. A downstream-side portion of the downstream-side bypass passage is connected with the downstream-side main exhaust passage at a position between the upstream-side main exhaust passages and the main catalyst converter. The shift valve device is disposed at a position from the upstream-side portions of the upstream-side main exhaust passages to a head flange fixed on a cylinder head of the internal combustion engine, and it is connected with the upstream-side portions of the upstream-side main exhaust valve device and upper portions of the bypass exhaust passages so as to be shiftable between a first shift position where the upstream-side main exhaust passages are closed and the bypass exhaust passages are opened and a second shift position where the upstream-side main exhaust passages are opened.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects, features and advantages of the present invention will become apparent as the description proceeds when taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic diagram showing an exhaust system of an internal combustion engine of a first embodiment according to the present invention;

FIG. 2 is a perspective view showing the exhaust system of the first embodiment;

FIG. 3 is a side view showing a shift valve device which is used in the exhaust system, seen along an arrow A1 in FIG. 2;

FIG. 4 is a perspective view showing a shift valve unit constituting the shift valve device shown in FIG. 3;

FIG. 5 is a cross sectional side view of the exhaust system of the first embodiment, taken along a line B-B in FIG. 4;

FIG. 6 is a cross sectional side view of an exhaust system of an internal combustion engine of a second embodiment according to the present invention; and

FIG. 7 is a cross sectional side view of an exhaust system of an internal combustion engine of a third embodiment according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Throughout the following detailed description, similar reference characters and numbers refer to similar elements in all figures of the drawings, and their descriptions are omitted for eliminating duplication.

Referring to FIGS. 1, 2 and 5 of the drawings, there is shown a first preferred embodiment of an exhaust system of internal combustion engine according to the present invention. The exhaust gas system is connected with first to fourth exhaust ports 1A to 1D, communicating with first to fourth cylinders #1 to #4 of cylinder of an internal combustion engine, formed in a cylinder head 1 to discharge and purify exhaust gas discharged from the engine.

The exhaust system includes four upstream-side main exhaust passages 2, a downstream-side main exhaust passage 3, upstream-side bypass exhaust passages 7 a to 7 d, downstream-side bypass exhaust passage 10, a main catalyst converter 5, a bypass main catalyst converter 9 and a shift valve device consisting of four shift valve units 6, one of which is shown in FIG. 4.

A head flange 11 with mounting tubes 2 a are fixed on the cylinder head 1 so that the mounting tubes 2 a can communicate with the first to fourth cylinders #1 to #4 through first to the fourth exhaust ports 1A to 1D, respectively. The mounting tubes 2 a constitute a part of the upstream-side main exhaust passages 2. Downstream-side portions of the mounting tubes 2 a are connected with upstream-side portions of the shift valve units 6, downstream-side portions of which are connected with the four upstream-side main exhaust passages 2, respectively. The four upstream-side main exhaust passages 2 are called as headers.

The downstream-side portions of the upstream-side exhaust main exhaust passages 2 are collected so as to form a collecting portion 4 thereof, which is connected with an upstream-side portion of the downstream-side main exhaust passage 3. A downstream-side portion of the downstream-side main exhaust passage 3 is connected with upstream-side portion of the main catalyst convert 5. A downstream-side portion of the main catalyst converter 5 is connected with not-shown exhaust pipes and a not-shown muffler.

On the other hand, upstream-side portions of the upstream-side bypass exhaust passages 7 a to 7 d are connected with bypass communicating ports 65, shown in FIG. 5, of the shift valve units 6, respectively. Downstream-side portions of the first and second bypass exhaust passages 7 a and 7 b is connected with a first intermediate passage 8 a at a junction P1, while downstream-side portions of the third and fourth bypass exhaust passages 7 c and 7 d is connected with a second intermediate passage 8 b at a junction P2. The first and second intermediate passages 8 a and 8 b are connected with a third intermediate passage 8 c at a junction P3. A downstream-side portion of the third intermediate passage 8 c is connected with an inlet portion of the bypass catalyst converter 9. An outlet port of the bypass catalyst converter 9 is connected, through the downstream-side bypass exhaust passage 10, with the downstream-side of the downstream-side main exhaust passage 3 at its position between the collecting portion 4 and the main catalyst converter 5.

The first to fourth upstream-side bypass passages 7 a to 7 d, the first to third intermediate passages 8 a to 8 c and the downstream-side bypass exhaust passage 10 correspond to bypass exhaust passages of the present invention, the first to fourth upstream-side bypass passages 7 a to 7 d and the first to third intermediate passages 8 a to 8 c correspond to upstream-side bypass passages of the present invention, and the downstream-side bypass exhaust passage 10 corresponds to a downstream-side bypass passage of the present invention.

The upstream-side main exhaust passages 2, the downstream-side exhaust passage 3, the upstream-side bypass passages and the downstream-side bypass passage are made of metal tubes with a circular cross section except the downstream-side portions, corresponding to the collecting portion 4, of the upstream-side main exhaust passages 2 having a cross section of a quarter round shape.

Incidentally, the tube forming one of the upstream-side bypass passages 8 c is provided with aperture 16 for receiving a not-shown oxygen sensor to measures the proportion of oxygen (O₂) in the exhaust gas.

The main catalyst converter 5 contains three-way catalyst and hydro carbon (HC) trap catalyst, having a volume considerably larger than that of the bypass catalyst converter 9 so that it can purify the exhaust gas discharged from the engine that is running at its full throttle position. Accordingly, it is mounted under a vehicle body floor 17, which causes the exhaust passages becomes long to communicate between the exhaust ports 1A to 1D and the main catalyst converter 5.

On the other hand, the bypass catalyst converter 9 contains a first catalyst 9 a and a second catalyst 9 b, which are monolithic catalyst carriers and are separated at an upstream side and at a downstream side therein. A space 9 c formed between the first catalyst 9 a and the second catalyst 9 b communicates with an exhaust gas recirculation valve 15 connected with a not-shown inlet manifold. Incidentally, the first and second catalysts 9 a and 9 b are preferably catalyst excel at cold activation, such as noble metal catalyst.

For a while immediately after the engine starts, the normal discharge amount of the exhaust gas is not so much. Accordingly, there is no need for the bypass catalyst converter 9 to have a large volume like the main catalyst converter 5. In addition, since the temperatures of the exhaust gas and the bypass catalyst converter 9 are low at that time, it is preferable for the bypass catalyst converter 9 to have the volume as small as possible in order to rise up its temperature as rapidly as possible. Further, it is also preferable for the upstream-side bypass exhaust passages 7 a to 7 d and the first to third intermediate exhaust passages 8 a to 8 c to have as short lengths as possible. Accordingly, the bypass catalyst carrier 9 is designed to be a small one sufficient to be arranged in an engine room 16 at the position closest to the exhaust ports 1A to 1D.

As shown in FIG. 2, the four shift valve units 6 are disposed in the upstream-side portions of the upstream-side main exhaust passages 2, being supported on the cylinder head 1 of the internal combustion engine through a head flange 11. They are aligned in line along a longitudinal direction of head flange 11 as shown in FIGS. 2 and 3, and they are controlled at the same time by one rotary shaft 64 actuated by an actuator 18. This actuator 18 is controlled based on temperature of the main catalyst converter 5, temperature of engine coolant, temperature of the exhaust gas or running time of the engine for example.

The shift valve units 6 are shiftable between a first shift position and a second shift position. In the first shift position, the upstream-side main exhaust passages 2 are closed and the upstream-side bypass exhaust passages 7 a to 7 d are opened, so that the exhaust ports 1A to 1D communicate with the upstream-side bypass exhaust passages 7 a to 7 d, respectively, being blocked off from the upstream-side main exhaust passages 2. In the second shift position, the upstream-side main exhaust passages 2 are opened, so that the exhaust ports 1A to 1D communicate with the upstream-side main exhaust passages 2 in addition to the upstream-side bypass exhaust passages 7 a to 7 d.

The shift valve units 6 are constructed as follows.

As shown in FIG. 5, each shift valve unit 6 has a housing 60 and a rotatable arm 63. The housing 60 has an inner space O, which communicates with the bypass communicating port 65 and a main communicating port 61 b formed on a valve opening portion 61 of the housing 60.

The bypass communicating port 65 is connected with the corresponding upstream-side bypass exhaust passages 7 a, 7 b, 7 c, 7 d, while the main communicating port 61 b is connected with the upstream-side main exhaust passage 2. A passage area of the former is set to be considerably smaller than that of the latter. The inner space O also communicates with the corresponding exhaust port 1A, 1B, 1C, 1D.

The valve opening portion 61 is provided with a seat portion 61 a shaped like a taper of a part of a spherical surface.

On the other hand, the rotatable arm 63 is located in the housing 60, and it is fixed to the rotary shaft 64 so that the rotatable arm 63 can be controlled to rotate between the first and second shift position, as respectively indicated by an alternate long and short dash line and a solid line, by and around the rotary shaft 64. The rotatable arm 63 is provided at a tip portion thereof with a valve element 62 that is swingable relative to the rotatable arm 63.

The valve element 62 consists of a bolt 62 c functioning as a valve stem, and a seat member 62 b provided with a seat portion 62 a. The seat portion 62 a has an outer surface shaped like a part of a spherical surface s that the seat portion 62 a can fittably contact with the seat portion 61 a of the valve opening portion 61 at the first shift position of the shift valve unit 6. Incidentally, the seat portion 62 a corresponds to a curved surface seat of the present invention.

The bolt 62 c penetrates a through-hole 63 a formed in the tip portion of the rotatable arm 63, and the seat member 62 b is screwed on a screw portion of the bolt 62 c in such a way that the bolt 62 c is swingably fixed to the rotatable arm 63. Both of a bolt head 62 d of the bolt 62 c and the seat member 62 b have a diameter larger than that of the through-hole 63 a of the rotatable arm 63.

This swingable support of the valve element 62 ensures the seat portion 62 a of the valve element 62 to be seated on the valve opening portion 61 at the first position, automatically aligning centers thereof. Differential pressure between the upstream-side and the downstream-side of the valve element 62 increases gas-tightness between the valve element 62 and the valve opening portion 61 at the first shift position.

An inner wall portion of the housing 60 is formed with a recess 6 b having a tapered inner wall portion 6 a for receiving the bolt head 62 d of the bolt 62 c when the rotatable arm 63 is placed at the second shift position. At this position, the contact of the bolt head 62 d and an inner surface of the recess 6 b prevents the valve element 62 from oscillating. This can remove adverse influences on the occurrence of noise and on the exhaust system.

The tapered inner wall portion 6 a enables the bolt head 62 d of the valve element 62 to smoothly be seated in the recess 6 b by sliding therealong when the rotatable arm 63 rotates toward the second shift position.

The operation of the exhaust system of the first embodiment will be described.

When the engine starts, its temperature does not reach normal running one and the exhaust system including the catalyst converters 5 and 9 are not warmed. In addition, the engine is normally driven not run at high engine speed. Consequently, the exhaust gas is discharged from the cylinders #1 to #4 to the inner space O of the shift valve units 6 through the exhaust port 1A to 1D and the mounting tubes 2 a. At this time, the shift valve units 6 are shifted to the first shift position, where the rotatable arm 63 is placed at the position indicated by the alternate long and short dash line in FIG. 5 to block the upstream-side main exhaust passages from the inner space O while the upstream-side bypass exhaust passages 7 a to 7 d are communicated with the inner space O.

Therefore, the exhaust gas from the cylinders #1 to #4 is introduced into the bypass catalyst converter 9 through the passages 7 a to 7 d and 8 a to 8 c. As these passages are set to be as short as possible, reduction in temperature of the exhaust gas is quite small. In addition, the bypass catalyst converter 9 has the small volume. Therefore, the catalyst in the bypass catalyst converter 9 is heated up in a short time to reach its activation temperature. Thus, the exhaust gas begins to be purified in short time after the engine starts. The exhaust gas discharged from the bypass catalyst converter 9 is introduced into the main catalyst converter 5 through the downstream-side bypass exhaust passage 10 and a part of the downstream-side main exhaust passage 3. It heats up the main catalyst converter 5 and then it is discharged in the atmosphere through the exhaust pipes and the muffler. As the main catalyst converter 5 has the large volume and the passages from the exhaust ports 1A to 1D thereto is long, the temperature of the catalyst in the main catalyst converter 5 does not reach its activation one for a while, but it gradually goes up.

In this operation at the first shift position, the upstream-side bypass exhaust passages 7 a to 7 b are joined step-by-step by using the passages 8 a to 8 c so as to prevent the exhaust gas from returning toward upstream side. Therefore, the reduction in temperature of the exhaust gas is suppressed.

After a while, when temperature of the exhaust system is heated up to that at which the catalyst in the main catalyst converter 5 can work sufficiently, the shift valve units 6 are shifted by the rotary shaft 64 driven by the actuator 18 to the second shift position. At this position, the rotatable arm 63 is placed at the position indicated by the straight line in FIG. 5 to communicate the inner space O with the upstream-side main exhaust passages 2 in addition to the upstream-side bypass exhaust passages 7 a to 7 d. Consequently, most of the exhaust gas is effectively purified by the main catalyst converter 5 and the rest thereof is additionally purified by the bypass catalyst converter 9. The flow amount of the exhaust gas in the bypass catalyst converter 9 is small, which extends the life span of he bypass catalyst converter 9.

The exhaust system of the first embodiment has the following advantages.

The exhaust system of the first embodiment enables the passages between the exhaust ports 1A to 1D and the bypass catalyst converter 9 to be shorter, which can shorten time for the bypass catalyst converter 9 to start purifying the exhaust gas.

The shift valve units 6 can be easily adapted for four-cylinder engines, six-cylinders or the others, only by connecting the necessary numbers thereof, and they can be controlled by one rotary shaft 64. The tubes of the passages 2 and 7 a to 7 b can be easily connected with the shift valve units 6.

The center-alignable valve element 62 can provide excellent gas-tightness, and the recess 6 b with the tapered inner wall portion 6 a can prevent oscillation of the valve element 62.

Next, an exhaust system of a second embodiment according to the present invention will be described.

The exhaust system of the second embodiment is constructed similarly to that shown in FIG. 1.

As shown in FIG. 6, in the exhaust system of the second embodiment, each shift valve unit 6 is integrally formed with a flange portion 6 c having a plurality of through-holes at upstream side thereof. The shift valve unit 6 is fixed to a cylinder head of an engine through a head flange by a plurality of bolts 12 passing through the through-holes formed on the flange portion 6 c.

The mounting tubes 2 a of the first embodiment are removed in the second embodiment. The other portion and parts are constructed similarly to those of the first embodiment.

The exhaust system of the second embodiment has the following advantages in addition those of the first embodiment.

The shift valve units 6 can be fixed on the cylinder head 1 with high accuracy, which improves rotational movement of a rotary shaft 64. In addition, they can be assembled with high rigidity.

Next, an exhaust system of a third embodiment according to the present invention will be described.

The exhaust system of the third embodiment is constructed similarly to that shown in FIG. 1. A plurality of shift valve units 6 are shiftable between a first shift position and a second shift position. In the first shift position, upstream-side main exhaust passages 2 are closed and upstream-side bypass exhaust passages 7 a to 7 d are opened, so that exhaust ports 1A to 1D communicate with the upstream-side bypass exhaust passages 7 a to 7 d, respectively, being blocked off from the upstream-side main exhaust passages 2. In the second shift position, the upstream-side main exhaust passages 2 are opened and the upstream-side bypass exhaust passages 7 a to 7 d are closed, so that the exhaust ports 1A to 1D communicate with the upstream-side main exhaust passages 2, being blocked off from the upstream-side bypass exhaust passages 7 a to 7 d.

As shown in FIG. 7, a rotatable arm 63 is installed at a bottom side of an inner space O of a shift valve unit 6. The rotatable arm 63 is placed in a slanted state as shown in FIG. 7 to avoid direct hit of the exhaust gas at the second shift position. A bolt 62 c has a bolt head 62 d having a tapered portion 62 e at an outer circumferential top portion thereof. The tapered portion 62 e is formed to have a partial spherical surface so that it can fit with a tapered inner wall portion 6 d formed on an inner surface of a housing 60 at an inlet port of a bypass communicating ports, where the tapered inner wall portion 6 d is also formed to have a partial spherical surface. The other portion and parts are constructed similarly to those of the first embodiment.

The exhaust system of the third embodiment has the following advantages in addition to those of the first embodiment.

At the second shift position, the tapered portion 62 e of the bolt head 62 d of a valve element 62 contacts with the tapered inner wall portion 6 d to ensure gas-tightness between the inner space O and the upstream-side bypass exhaust passages 7 a to 7 d. This prevents the oscillation of the valve element 62 at the second shift position.

In addition, at this second shift position, the exhaust gas does not flow into the bypass catalyst converter 9, which can extend the life span thereof.

The valve element 62 and the rotatable arm 63 can be avoided from direct hit of the exhaust gas at the second shift position because of the inclination thereof. This can extend the life span thereof.

While there have been particularly shown and described with reference to preferred embodiments thereof, it will be understood that various modifications may be made therein, and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

The number of cylinders of the engine may be set appropriately, and engine types may be also designed appropriately.

The downstream side main exhaust passage may be one or more than one. The main catalyst converter 5 may be one or more than one.

The head flange 11 and the housings 60 of the shift valve device may be formed as one unit, for example by employing castings.

The shift valve units 6 may be formed as one unit instead of separated ones.

In the embodiments, a gasket is not provided, but it is preferable to provide one between the cylinder head 1 and the head flange 11.

The entire contents of Japanese Patent Application No. 2007-299300 filed Nov. 19, 2007 are incorporated herein by reference. 

1. An exhaust system of an internal combustion engine comprising: a main catalyst converter for purifying exhaust gas discharged from an internal combustion engine; a bypass catalyst converter for purifying the exhaust gas converter, the bypass catalyst converter having a volume smaller than a volume of the main catalyst converter, the bypass catalyst converter being arranged at a position closer to exhaust ports of the internal combustion engine relative to a position of the main catalyst converter; upstream-side main exhaust passages having upstream-side portions and downstream-side portions having a collecting portion; a downstream-side main exhaust passage having an upstream-side portion and a downstream-side portion, the upstream portion thereof being connected with the collecting portion of the upstream-side main exhaust passages, and the downstream-side portion thereof being connected with the main catalyst converter; bypass exhaust passages including upstream-side bypass passages and a downstream-side bypass passage so that the bypass catalyst converter is disposed between the upstream-side bypass passages and the downstream-side bypass passage, a downstream-side portion of the downstream-side bypass passage being connected with the downstream-side main exhaust passage at a position between the upstream-side main exhaust passages and the main catalyst converter; and a shift valve device that is disposed at a position from the upstream-side portions of the upstream-side main exhaust passages to a head flange fixed on a cylinder head of the internal combustion engine, the shift valve device being connected with the upstream-side portions of the upstream-side main exhaust valve device and upper portions of the bypass exhaust passages so as to be shiftable between a first shift position where the upstream-side main exhaust passages are closed and the bypass exhaust passages are opened and a second shift position where the upstream-side main exhaust passages are opened.
 2. The exhaust system of the internal combustion engine according to claim 1, wherein the shift valve device is disposed in the upstream-side portions of the upstream-side bypass passages which are connected with the head flange fixed on the cylinder head.
 3. The exhaust system of the internal combustion engine according to claim 2, wherein the shift valve device has a plurality of shift valve units that are arranged in line along a longitudinal direction of the cylinder head, the shift valve units being shiftable by one shaft actuated by an actuator.
 4. The exhaust system of internal combustion engine according to claim 3, wherein the shift valve device includes a housing formed with a valve opening portion, and a rotatable arm rotatable relative to the valve opening portion, the valve opening portion having a seat portion shaped like a taper and being communicatable with the upstream-side portions of the upstream-side main exhaust passages, and the rotatable arm being provided with a valve element having a curved surface seat that is swingable relative to the rotatable arm so that the curved surface seat and the seat portion can be aligned with each other when the shift valve device is shifted to the first shift position.
 5. The exhaust system of internal combustion engine according to claim 4, wherein the valve element is connected with the rotatable arm though a valve stem thereof, and wherein the valve element contacts with an inner surface of the housing so that the valve element is prevented from oscillating due to contact of the valve element and the inner surface when the shift valve device is at the second shift position.
 6. The exhaust system of internal combustion engine according to claim 5, wherein the bypass catalyst converter is disposed in an engine room, and the main catalyst converter is mounted under a vehicle floor.
 7. The exhaust system of the internal combustion engine according to claim 1, wherein the shift valve device has a plurality of shift valve units that are arranged in line along a longitudinal direction of the cylinder head, the shift valve units being shiftable by one shaft actuated by an actuator.
 8. The exhaust system of internal combustion engine according to claim 7, wherein the shift valve device includes a housing formed with a valve opening portion, and a rotatable arm rotatable relative to the valve opening portion, the valve opening portion having a seat portion shaped like a taper and being communicatable with the upstream-side portions of the upstream-side main exhaust passages, and the rotatable arm being provided with a valve element having a curved surface seat that is swingable relative to the rotatable arm so that the curved surface seat and the seat portion can be aligned with each other when the shift valve device is shifted to the first shift position.
 9. The exhaust system of internal combustion engine according to claim 8, wherein the valve element is connected with the rotatable arm though a valve stem thereof, and wherein the valve element contacts with an inner surface of the housing so that the valve element is prevented from oscillating due to contact of the valve element and the inner surface when the shift valve device is at the second shift position.
 10. The exhaust system of internal combustion engine according to claim 1, wherein the shift valve device includes a housing formed with a valve opening portion, and a rotatable arm rotatable relative to the valve opening portion, the valve opening portion having a seat portion shaped like a taper and being communicatable with the upstream-side portions of the upstream-side main exhaust passages, and the rotatable arm being provided with a valve element having a curved surface seat that is swingable relative to the rotatable arm so that the curved surface seat and the seat portion can be aligned with each other when the shift valve device is shifted to the first shift position.
 11. The exhaust system of internal combustion engine according to claim 10, wherein the valve element is connected with the rotatable arm though a valve stem thereof, and wherein the valve element contacts with an inner surface of the housing so that the valve element is prevented from oscillating due to contact of the valve element and the inner surface when the shift valve device is at the second shift position.
 12. The exhaust system of internal combustion engine according to claim 1, wherein the bypass catalyst converter is disposed in an engine room, and the main catalyst converter is mounted under a vehicle floor.
 13. The exhaust system of internal combustion engine according to claim 1, wherein the shift valve device and the head flange fixed on the cylinder head are one of one unit thereof and integral parts thereof.
 14. The exhaust system of internal combustion engine according to claim 13, wherein the shift valve device has a plurality of shift valve units that are arranged in line along a longitudinal direction of the cylinder head, the shift valve units being shiftable by one shaft actuated by an actuator.
 15. The exhaust system of the internal combustion engine according to claim 14, wherein the shift valve device includes a housing formed with a valve opening portion, and a rotatable arm rotatable relative to the valve opening portion, the valve opening portion having a seat portion shaped like a taper and being communicatable with the upstream-side portions of the upstream-side main exhaust passages, and the rotatable arm being provided with a valve element having a curved surface seat that is swingable relative to the rotatable arm so that the curved surface seat and the seat portion can be aligned with each other when the shift valve device is shifted to the first shift position.
 16. The exhaust system of internal combustion engine according to claim 15, wherein the valve element is connected with the rotatable arm though a valve stem thereof, and wherein the valve element contacts with an inner surface of the housing so that the valve element is prevented from oscillating due to contact of the valve element and the inner surface when the shift valve device is at the second shift position.
 17. The exhaust system of internal combustion engine according to claim 16, wherein the bypass catalyst converter is disposed in an engine room, and the main catalyst converter is mounted under a vehicle floor.
 18. The exhaust system of internal combustion engine according to claim 13, wherein the shift valve device includes a housing formed with a valve opening portion, and a rotatable arm rotatable relative to the valve opening portion, the valve opening portion having a seat portion shaped like a taper and being communicatable with the upstream-side portions of the upstream-side main exhaust passages, and the rotatable arm being provided with a valve element having a curved surface seat that is swingable relative to the rotatable arm so that the curved surface seat and the seat portion can be aligned with each other when the shift valve device is shifted to the first shift position.
 19. The exhaust system of internal combustion engine according to claim 18, wherein the valve element is connected with the rotatable arm though a valve stem thereof, and wherein the valve element contacts with an inner surface of the housing so that the valve element is prevented from oscillating due to contact of the valve element and the inner surface when the shift valve device is at the second shift position.
 20. The exhaust system of internal combustion engine according to claim 12, wherein the bypass catalyst converter is disposed in an engine room, and the main catalyst converter is mounted under a vehicle floor. 